Magnitude scaling of induced earthquakes

• Measured earthquake magnitudes, particularly for small events, are not definitive.
• We explore the scaling of earthquake magnitude from induced earthquakes, with focus on the potential impact in hazard assessment.
• We show that when recomputing magnitudes to a common scale, their relation to the moment magnitude is well-defined.
• Two subsets are apparent, indicating different source or attenuation properties between the sites within each subset.

Presented are the results of an earthquake magnitude homogenisation exercise for several datasets of induced earthquakes. The result of this exercise is to show that homogeneous computation of earthquake moment- and local-magnitude is useful in hazard assessment of Enhanced Geothermal Systems (EGSs). Data include records from EGSs in Basel (Switzerland), Soultz (France) and Cooper Basin (Australia); natural geothermal fields in Geysers (California) and Hengill (Iceland), and a gas field in Roswinkel (Netherlands). Published catalogue magnitudes are shown to differ widely with respect to Mw, with up to a unit of magnitude difference. We explore the scaling between maximum-amplitude and moment-related scales. We find that given a common magnitude definition for the respective types, the scaling between moment- and local-magnitude of small earthquakes follows a second-order polynomial, consistent with previous studies of natural seismicity. Using both the Southern-California ML scale and a PGV-magnitude scale (Mequiv) determined in this study, we find that the datasets fall into two subsets with well-defined relation to Mw: Basel, Geysers and Hengill in one and Soultz and Roswinkel in another (Cooper Basin data were not considered for this part of the analysis because of the limited bandwidth of the instruments). Mequiv is shown to correlate 1:1 with ML, albeit with region-specific offsets, while the distinct subsets in the Mequiv to MW scaling leads us to conclude that source and/or attenuation properties between the respective regions are different.